Various techniques for multiplexing, onto image information, a different kind of information have been proposed as copyright protection, forgery prevention of banknotes, protection of secret information, and transmission methods of text data and sound data including audio data.
In recent years, a technique called digital watermarking, which multiplexes, onto image information such as a photo, painting, or the like, additional information including the copyright holder name, permission/denial of use, and the like so as to be hard to visually detect, and distributes such information via a network such as the Internet or the like, has prevailed.
For example, “Digital Watermarking and its Evaluation items”, IIEEJ Journal, Vol. 27, No. 5, p. 483, 1998 has announced various digital watermarking techniques. Information multiplexing methods are roughly categorized into a method of embedding information in the frequency domain and a method of embedding information in the real space domain. When information is embedded in the frequency space, image information is converted into that in the frequency domain using given means such as Fourier transformation or the like, and information is then multiplexed using frequency components, phase components, or the like. On the other hand, when information is embedded in the real space domain, information is multiplexed using the value of the least significant bit and quantization errors.
Current information multiplexing mainly aims at copyright protection of digital images and the like, and data to be multiplexed are often limited to the copyright holder name, place, date of creation, and the like.
On the other hand, as methods of mixing an image containing a text document or the like and another kind of variable-length information such as audio data or the like in a single medium, a method of describing information in a blank, a method of describing information using ink which is quite different from inks that form an image, and a method of mixing a code having a shape independently of the image information as information although ink that forms the image is used are known.
As another application field of such technique, as image forming apparatuses such as copying machines, printers, and the like attain higher image qualities, a technique for embedding additional information into an image to specify the output device model and model number from the image output onto a recording sheet for the purpose of preventing securities such as banknotes, revenue stamps, and the like from being forged is known.
For example, a technique for multiplexing information by embedding additional information in the high-frequency range of color difference and saturation components with low visual sensitivity has been known.
However, the conventional multiplexing technique suffers the following problems.
When information is multiplexed using the frequency domain, since frequency components that each image requires differ depending on images, the data size of additional information that can be multiplexed while suppressing deterioration of image quality is determined for each image of interest. Hence, when the additional information size is large, the frequency domain required for multiplexing increases, and it is often impossible to multiplex all pieces of information in some images.
When information is multiplexed using the real space domain, additional information embedded into a region which is easy to visually detect causes deterioration of image quality. On the other hand, when additional information is embedded into a region which is hard to decode due to influences of surrounding image information, the embedded information cannot be used. That is, a region in which additional information can be embedded is limited. As a result, the information size that can be multiplexed is limited, and all pieces of information cannot often be multiplexed.
The method of describing additional information in a blank is not desirable since a region other than an image is required. The method of using special ink is not desirable since ink other than those used in a normal print process is required and, hence, cost may rise.
FIG. 21 depicts an example of the additional information embedding method. As shown in FIG. 21, image information A and additional information B are multiplexed via an adder 1201 to generate multiplexed information C. The adder 1201 may add information in the real space of the image information A or may transform the image information A into that in the frequency domain using, e.g., Fourier transformation, or the like and may then synthesize the additional information B in the high-frequency range of the transformed information.
If it is possible to distribute the multiplexed information C generated in this way without any image processes such as various filtering processes and the like or any encoding processes such as lossy compression or the like, it is possible to decode the additional information B from the multiplexed information C. Also, image information, which is distributed on, e.g., the Internet, can also be decoded via digital filters for improving image quality such as edge emphasis, smoothing, and the like, as long as it has some noise resilience.
Assume that an image forming apparatus has only expression performance as low as two to several gray levels per color. In recent years, ink-jet printers can express several gray levels per color using inks with lower dye densities or by variably controlling the dot sizes to be output. However, such printers cannot express a photo-quality image unless a pseudo halftone process is used.
FIG. 22 depicts an example that executes a pseudo halftone process upon multiplexing additional information. That is, in addition to the arrangement shown in FIG. 21, the multiplexed information is converted into quantized information D by a pseudo halftone process 1301, and the converted information is printed out onto a recording sheet by a printer output 1302, thus obtaining printed information E that has deteriorated considerably. Hence, decoding additional information from information on a recording sheet for the purpose of preventing forgery amounts to decoding additional information from the printed information E after the series of processes shown in FIG. 22. However, the image information changes to a large extent by the two processes, i.e., the pseudo halftone process 1301 and printer output 1302. Therefore, it is very difficult to multiplex additional information so as not to be visually detectable, and to successfully decode the multiplexed additional information B from the printed information E.
The aforementioned multiplexing technique embeds information in the high-frequency region of an image. When the subsequent pseudo halftoning uses error diffusion, the frequency range of the additional information is buried under that of a texture produced by error diffusion due to the high-pass filter characteristics unique to error diffusion, and it becomes harder to decode the additional information. Also, in order to accurately decode the additional information from the printed information, a scanner device with very high precision is required.
That is, when the pseudo halftone process must be done, the multiplexing method shown in FIG. 13 is not suitable. In other words, an additional information multiplexing method that fully utilizes the characteristics of the pseudo halftone process is needed.
There are techniques combining multiplexing of additional information and redundancy of the pseudo halftone process.
In the techniques, upon binarization using ordered dithering, additional data is mixed into an image signal by selecting one of dither matrices indicating an identical gray level. However, in ordered dithering, it is difficult to output a photo-quality image unless a printer having a high resolution and very high mechanical precision is used. Small mechanical precision errors are produced as low-frequency noise such as horizontal stripes or the like, and are easily visually detectable on paper. When the dither matrices are periodically changed, a specific frequency range generated by a regular dither pattern is disturbed, thus adversely influencing image quality. Also, the decoder must decode additional information by estimating the dither matrix used in binarization while the pixel values of image information as an original signal are unknown and, hence, it is hard to attain accurate decoding.
The second technique has proposed a method of multiplexing additional information using a color dither pattern method. In the second technique, deterioration of image quality upon switching the dither matrices cannot be avoided as in the first technique. Compared to the first technique, a larger number of pieces of additional information can be multiplexed, but the color tincture changes since the sequence of color components changes, and image quality deteriorates considerably in a portion with little grayscale change. Also, decoding becomes more harder to attain.
In any case, these methods that change the dither matrices suffer the problem that decoding is hard to attain despite the fact that image quality deteriorates considerably.
Method of embedding large-size information such as audio information in an image is known. In this method, audio information is converted into a dot code so-called a two-dimensional barcode, and the barcode is printed on a blank of an image or inside the image. However, this method neither multiplexes a dot code as additional information on image information nor appends additional information (dot code) so as to be hard to visually detect. This reference describes only one example for embedding a code using transparent paint as a devise for making a dot code hard to visually detect. However, since special ink is required, the cost inevitably increases, and the quality of an image printed out on a recording sheet deteriorates.